Dentifrices have heretofore been used for prevention of caries, prevention of bad breath, or oral cleansing. Such dentifrices, however, have poor usability because of their need for using a brush. Mouthwashes excellent in convenience have therefore become more popular in lifestyles and been widely used. These mouthwashes, however, are used for the purpose of keeping the mouth clean in such a way as to suppress the growth of harmful bacteria or prevent bad breath. Nonetheless, with changes in feelings for cleanliness or health, the mouthwashes have been demanded in recent years to have various functions including tooth whitening, tooth shining, and active contribution to the repair or health of teeth through remineralization action. In order to satisfy these demands, studies have been made on a mouthwash (patent documents 2 to 4) supplemented with hydroxyapatite (patent document 1), which is used as dentifrices and known to have the effects of promoting the remineralization of teeth and of whitening teeth.
Due to tendencies toward insufficient intakes of calcium, the intakes of calcium in necessary amounts in daily diets has been demanded in recent years. For example, foods such as milk, yoghurt, and refreshing beverages have been calcium-fortified actively. Upon addition of water-soluble calcium such as calcium lactate or calcium chloride as a calcium ingredient to dairy products such as milk or soy milk, the calcium ions react with proteins during thermal sterilization to precipitate the proteins. This phenomenon deteriorates taste and flavor and adversely affects the original taste of the foods. For these reasons, there is a ceiling to the addition of the water-soluble calcium. Recent studies have therefore been directed to, for example, a method which involves preparing fine particles of calcium carbonate or calcium phosphate, which is water-insoluble calcium, and dispersing the fine particles in foods by the addition of a dispersant. Unfortunately, calcium carbonate cannot be added in large amounts due to its bitterness. Again, the intakes of calcium in sufficient amounts cannot be achieved. Accordingly, if the dispersibility of calcium phosphate, particularly, a bone component hydroxyapatite, as a calcium ingredient can be stably maintained in water, various problems brought about by water-soluble calcium or calcium carbonate can be solved.
The water-insoluble calcium such as calcium phosphate, however, usually has a high specific gravity of 1.5 or larger and therefore, is easily precipitated in water. In addition, the hydroxyapatite, in spite of being fine particles (primary particles of 0.1 μm or smaller), is easily aggregated through van der Waals' force, ionic charge, or the like and therefore usually tends to form an aggregate having a particle diameter on the order of 4 to 100 μm. Since such an aggregate is significantly low dispersible in water, slurry has poor storage stability and is disadvantageously separated into water and a precipitate (hydroxyapatite aggregate) in a short time.
Thus, methods for enhancing the dispersibility of hydroxyapatite and suppressing its sedimentation (patent documents 5 to 8) have been studied as to oral compositions. Also, methods for improving the dispersibility of an insoluble substance such as an abrasive contained in mouthwashes or the like and suppressing its sedimentation (patent documents 9 and 10) are known. As for liquid food compositions, there have been proposed, for example: a food excellent in dispersion stability containing water-insoluble mineral and enzymatically degraded lecithin (patent document 11); a method for preventing calcium from being sedimented and preventing the flavor and quality of a liquid food product from being deteriorated, comprising adding insoluble calcium, lecithin, and polyglycerin fatty acid ester to the liquid food product (patent document 12); and a food composition, such as calcium-fortified milk, which is excellent both in particle homogeneity and dispersibility and in thermal stability, rarely causes sedimentation, and has favorable flavor by virtue of fine particles of calcium phosphate (patent document 13).
These proposed methods, however, still fail to offer favorable dispersion stability of calcium phosphate and cannot always produce sufficient oral compositions such as mouthwashes, liquid dentifrices, and oral detergents or liquid food compositions such as milk, soy milk, yoghurt, and refreshing beverages in which the calcium phosphate is not sedimented for a long period. Thus, an effective method has been demanded.